How mapping neurons could reveal how experiences affect mental wiring

This article was taken from the July 2012 issue of Wired
magazine. Be the first to read Wired's articles in print before
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No road, no trail can penetrate this
forest. The long and delicate branches of its trees lie everywhere,
choking space with their exuberant growth. No sunbeam can fly a
path tortuous enough to navigate the narrow spaces between these
entangled branches. All the trees of this dark forest grew from 100
billion seeds planted together. And, all in one day, every tree is
destined to die.

This forest is majestic, but also comic and even
tragic. It is all of these things. Indeed, sometimes I think it is
everything. Every novel and every symphony, every cruel murder and
every act of mercy, every love affair and every quarrel, every joke
and every sorrow -- all these things come from the
forest.

You may be surprised to hear that it fits in a
container less than 30 centimetres in diameter. And that there are
seven billion on this Earth. You happen to be the caretaker of one,
the forest that lives inside your skull. The trees of which
I speak are those special cells called neurons. The mission of
neuroscience is to explore their enchanted branches -- to tame the
jungle of the mind. Neuroscientists have eavesdropped on its
sounds, the electrical signals inside the brain. They have revealed
its fantastic shapes with meticulous drawings and photos of
neurons. But from just a few scattered trees, can we hope to
comprehend the totality of the forest?

In the 17th century, the French philosopher Blaise
Pascal confessed that he was terrified by the vastness of the
universe. Pascal meditated upon outer space, but we need only turn
our thoughts inward to feel his dread. Inside every one of us lies
an organ so vast in its complexity that it might as well be
infinite.

As a neuroscientist I have come to
know first-hand Pascal's dread. I have also felt embarrassment.
Sometimes I speak to the public about the state of our field. After
one such talk I was pummelled with questions. What causes
depression and schizophrenia? What is special about the brain of an
Einstein or a Beethoven? How can my child learn to read better? As
I failed to give satisfying answers, I could see faces fall. I
finally apologised. "I'm sorry," I said. "You thought I'm a
professor because I know the answers. Actually I'm a professor
because I know how much I don't know."

Studying an object as complex as the
brain may seem almost futile. The brain's billions of neurons
resemble trees of many species and come in many fantastic shapes.
Only the most determined explorers can hope to capture a glimpse of
this forest's interior, and even they see little, and see it
poorly. It's no wonder that the brain remains an enigma. My
audience was curious about brains that malfunction or excel, but
even the humdrum lacks explanation. Every day we recall the
past, perceive the present, and imagine the future. How do our
brains accomplish these feats? It's
safe to say that nobody really knows.

Daunted by the brain's complexity,
many neuroscientists have chosen to study animals with drastically
fewer neurons than humans. The worm lacks what we'd call a brain.
Its neurons are scattered throughout its body rather than
centralised in a single organ. Together they form a nervous system
containing a mere 300 neurons. That sounds manageable. Every neuron
in this worm has been given a unique name and has a characteristic
location and shape. Worms are like precision machines mass-produced
in a factory: each one has a nervous system built from the same set
of parts, and the parts are always arranged in the same
way.

What's more, this standardised nervous system has
been mapped completely. The result is something like the flight
maps we see in the back pages of airline magazines. The four-letter
name of each neuron is like the three-letter code for each of the
world's airports. The lines represent connections between neurons,
just as lines on a flight map represent routes between cities. We
say that two neurons are "connected" if there is a small junction,
called a synapse, at a point where the neurons touch. Through the
synapse one neuron sends messages to the other.

Engineers know that a radio is constructed by wiring
together electronic components like resistors, capacitors and
transistors. A nervous system is likewise an assembly of neurons,
"wired" together by their slender branches. That's why the map
was originally called a wiring diagram. More recently, a new
term has been introduced -- connectome.

This word invokes not electrical
engineering but the field of genomics. DNA is a long molecule
resembling a chain. The individual links of the chain are small
molecules called nucleotides, which come in four types denoted by
the letters A, C, G and T. Your genome is the entire sequence of
nucleotides in your DNA, or equivalently a long string of letters
drawn from this four-letter alphabet. With three billion letters it
would be a million pages long if printed as a book. In the same
way, a connectome is the totality of connections between the
neurons in a nervous system. The term, like genome, implies
completeness. A connectome is not one connection, nor even many. It
is all of them. In principle, your brain could also be summarised
by a diagram that is like the worm's, though much more complex.
Would your connectome reveal anything interesting about
you?

The first thing it would reveal is that you are
unique. You know this, of course, but it has been surprisingly
difficult to pinpoint where, precisely, your uniqueness resides.
Your connectome and mine are very different. They are not
standardised like those of worms. That's consistent with the idea
that every human is unique in a way that a worm is not.

Differences fascinate us. When we ask how the brain
works, what mostly interests us is why the brains of people work so
differently. Why can't I be more outgoing, like my extroverted
friend? Why does my son find reading more difficult than his
classmates do? Why is my teenage cousin starting to hear imaginary
voices? Why is my mother losing her memory? Why can't my spouse (or
I) be more compassionate and understanding?

I propose a simple theory: minds
differ because connectomes differ. Personality and IQ might also be
explained by connectomes. Perhaps even your memories, the most
idiosyncratic aspect of your personal identity, could be encoded in
your connectome.